Process for the decomposition of siliceous aluminous minerals



July 2, 1935.

E. WIEDBRAUCK ET AL v2,006,851

PROCESS FOR THE DECOMPOSITION OF SILICEOUS ALUMINOUS MINERALS Filed Aug. 5, 1952 who 10. 20 a 40 so 0 10 a0 90 50 Patented July 2, 1935 V UNITED- i srATEs SITION SILICEOUS ALUMINOUS'll/IINERALS Erich Wi'edbrauck and Karl Biifche', Essen-Ruhr;

Germany,'assignorsfby' mesne assignments, to v Firm ,Th. Goldschmidt A.'-'G.-,' Essen-Ruhr,

Germany Application A ust 5, 1932, SerialNo. 627,593 I In GermanyAugust 5, 1931 r'onims. (01. 23-143 7 k I Attempts to decompose more abundant and more resistant minerals, such as clays, kaolin, leucite, high silica bauxite, etc., by themethods used-with these readily soluble mineral, have This invention relates to a process of;decomposing silicate mineralscontaining alumina for the production of alumina.

In the production of alumina for the manu-,

facture of aluminum or aluminum compounds, it is usual to employ bauxites low in silica., From these bauxites, pure alumina is generally obtained by alkali methods, such as the well known Bayer method employing caustic soda solution or the Le Chatelier method using sodium carbonate. To a less extent, pure alumina is obtained from minerals high in silica, such as the kaolins, clays, leucites, high silica bautxites, etc. These materials occur in nature moreextensively than high grade bauxite. The trouble with these siliceous minerals resides in thehigh silica content and the difliculty of separating alumina from silica in a complete and economical manner. Attempts have long been made to obviate these difliculties and render aluminum silicates available for the aluminumindustry. For example, attempts have been made to de-, compose these mineralswith stronggacidSLsuch as sulfuric acid, hydrochloric acid and nitric acid, orwith their acid salts. The ammonium salts have been used. 7 The diificulty arises that although alumina can be separated from silica in these methods, the iron, which isalways present, also passes into solution and its separation from the alumina salts is difiicult and costly. It is, moreover, difiicult to recover the acids used for reuse, from these solutions. Any way of recovery involves extensive concentration, and thermal decomposition of the salts. p 7 I It has been attempted to decompose certain, aluminum silicates with weak acids, as for example, with sulfurous ,acid. The attempts, however, have been limited to the treatment ofsill-1 cates of the sodalite or nepheline syenite type; materials easily soluble in acids and containing' besides alumina, silica and iron, considerable quantities of alkali compounds or alkaline earth compounds. Decomposition of these ma terials is comparatively easy. I Treatnientwith weak suliurous acid solution, even'at'the ordinary temperature, sufiices to dissolve out mosto'f the bases present in these minerals. Sometimes operation was warmpat temperatures not .be-

from the heat evolved by the chemical-reaction. With'these methods difficulty arises in the separation of thesilica which goes'into solution, to

gether with the alumina, this,necessitating trou- 5 blesome. heating and filtering steps. g

yond 40 C.; that is, at temperatures resulting given unsatisfactory results.

These materials usually contain only small proportions of the readily attackable complex double silicates of alumina and alkali (aluminosilicates) of the type of those constituting sodalite and nephelinesyenite; they carry but little alkali.

The

quantity of alumina brought into'solution by a treatment with suliurous acidunder conditions hitherto known in any practically applicable time does not Warrant the operation.

As we have found, it is only by using specific methods of operation, including higher temperatures, that satisfactory results can be obtained.

It hasbeen found that not all high temperatures are applicable since in manycases the aluminum sulfite solutions obtained undergo decomposition with re-separation of some of the alumina originally dissolved, this representing a loss of alumina.

The time factor, the agefof,

thesolution, is also important. In solutions made by the hereinafter described method and carrying dissolved aluminum sulfite, it is foundthat definite decomposition temperatures occur; these, depending on the alumina concentration and the S02 concentration, OISOz pressure. The decomposability of these solutions with a rise in tem-. prature increases with an increase in A1203 con.

tent and diminishes with an increase in S02 content, as shown the, following table:

If an aluminum sulfite solution be heated above its decomposition temperature, it deposits alumina in the form of a basic aluminum sulfite. The re action temperaturesin the present operation must therefore be kept below this decomposition temperature.

Mme

In the accompanying drawing, we have shown, graphically, certain of the described relations.

Figure 1 illustrates the behavior of an aluminum sulfite solution at various temperatures. The curve illustrates the relation of the alumina concentration of'the solution to the decomposition temperature of aluminum sulfite solutions. While aluminum sulfite solutionseven those of high concentration-are stable below 0., the

drop in the curve above that pointindicates a rapid decrease of the amount ofaluminaheld'in solution as the temperature rises above 70 0.,

even when the S02 is considerable excess. Above 100 0., only small amountsof A1202 remain in stable solution.

The curve in Fig. l was obtained by heating various samples of an aluminum sulfitejsolution, containing 40 grams per liter A1203 and from25 to 30 per cent S02 per liter, to the varioustemperatures indicated on the plot, then analyzing the resulting solution for its alumina content. 'Ihe samples were maintained at the temperatures indicated for a'period of ihours. Itis shown by the plotthat these samples remained stable durin this-heatingperiod-up'to temperatures of about 70 C. Athigher temperatures a decomposition occurred, part of the alumina precipitating out ofsolution, until at temperatures of 100- C. only smallamounts remained in solution.

Furthermore, it has been found that the stability of aluminum sulfite in solution depends largelyon the concentration of the sulfurous acid present. This in turn depends on the concentration-of S02 in the system.

'Figure 2 of the drawing shows the efiect of the concentration of sulfurous acid in the solution with which clay is treated upon the solubility of the'alumina or the amount of alumina extracted fromthe clay. The solubility values of A1202 are plotted as ordinates and the S02 concentrations as abscissae. The curve shows clearly that in extracting alumina from clay in sulfurous acid solution A1203 concentrations exceeding 20 grams per liter of solution, which is the lowest limit of practical utility, cannot be obtained except with $0 concentrations exceeding 10 per cent of the aqueous solution. Increase of the S02 increases the solubility of the alumina up to a maximum of about 68 grams per liter. It is thus clearly evident that in decomposing the clay it is desirable'to operate with high concentrations of S02.

The curve in Fig. 2 was obtained by treating test samples of clay for 12 hours with solutions of S02 of varying concentration, as indicated on the plot, and then analyzing the resulting solutions for their content of A1202. The treatment was conducted in each instance at a temperature of 70 C. and the clay was kept in excess during the treatment. At temperatures other than 70 C. the curvesobtained are very similar in character.

As regards the time required for-decomposition of the clay, the general rule thattime-decreases with increasingtemperature is valid.

We have found that agitation ofrthe clay during its decomposition influences the stability of the aluminum sulfite solution extract and also influences the time required for decomposition of the clay. While agitation accelerates the decomposition of the clay, if the agitation is too extract are obtained by slow agitation during the treatment of the clay.

that an adequate amount of S02 be present. In

practice the sulfurous acid may be added in proportion as'it is consumed; as by leading in S02 gas. :Sodoing, it is possible to effect decomp0sition of the clay without pressure and without using a-great excess of sulfurous acid. It is however possibleto add the total amount of the S02 at the start-of the reaction. In such case, it is necessary to use closed vessels and to work under pressure. In; order to avoid decomposition of the aluminum sulfite solution extract and to obtain .high yields of alumina in highly concentrated solutions with a relatively short time needed for decomposition'of the clay, it is advantageous to use sulfurous acid in excessby working under high pressures and at temperatures at which aluminum sulfite solutions are stable. An excess of S02 tends to prevent decomposition of the aluminum sulfite solutions.

In practicing the present invention, the clay or other siliceous aluminous mineral is usually first calcined in order to remove water of hydration'and afterwards ground.- The ground calcinedmaterial is treated with an aqueous solution of sulfurous acid at a moderately high temperature and under superatmospheric pressure. The sulfurous acid is best employed in excess and works as a supersaturated solution. In thus treating the clay, the alumina and iron contents are dissolved while the silica remains substantially undissolved. So working, alumina is extracted from the clay in yields up to 85 per cent and=the solution extract reachesconcentrations up to-50 grams A1202 per liter. The solution is separated from the siliceous residue or gangue by filtration. The aluminum salts of the weak acid being more readily decomposable by hydrolysis than the ironsalts of the same acids, heating the solution extracts obtained in decomposingthe-clay, which extracts contain both the aluminum and the iron, if any, of the clay, effects a direct precipitation of the aluminum while iron salts remain in solution. The present process gives a direct separation of iron,thereby obviating an objection tomost of the'prior art methods in which the alumina recovered must be subjected to a troublesome after treatment to get rid of iron. Upon filtration, which is advantageously carried out under exclusion of air, the precipitated alumina is free from iron. Itis a particular advantage of the process described that,rasia.result of thehigh alumina concentrationsobtained in extracting the clay, the subsequently precipitated alumina compound is also nearlyireeof. silica. The product of the process maybe worked up in known ways to produce the pure. alumina or aluminum salts- ;,Belo'w'are given three specific examples of our processinvention.

' Example I fThe decomposition of calcined'aluminous minerals by sulfurous acid at atmospheric pressure is eifected as follows:

AJead-lined stirring-vessel'is charged with 25 liters of water and with 3.2 kilograms of calcined and ground clay containing 34.8 per cent of A1202. A weak currentofSoz gasis passed into the vessel with slow stirring of the contents, the temperature being kept at about 55 C. The solution in the reaction mixture contains, at the end of 3 hours, 0.75 per cent of A1203 and 2.4 per cent of S02 12 hours, 1.55 per cent of A1203 and 6.25 per cent of S02 24 hours, 2.55 per cent of A1203 and 8.30 per cent of S02 36 hours, 2.85 per cent of A1203 and 9.65 per cent of S02 42 hours, 3.00 per cent of A1203 and 10.00 per cent of S02 The yield of alumina is about 70 per cent. The solution is filtered oif from the undissolved residue of silica and is treated for the production of basic aluminum sulfites.

If an accelerated reaction be desired, the operation may be performed in a closed stirring apparatus under pressure such as 1 to atmospheres and a somewhat increased temperature (e. g. 50-70 0.), by introducing S02 gas continuously under pressure into the autoclave. At 2 atmospheres pressure and a temperature of 55 C. the extraction time amounts to 28 hours only and at a pressure of 4 atmospheres and 60C. to 20 hours only.

Example II The decomposition of aluminous materials with an excess of sulfurous acid at a superatmospheric pressure is performed in the following manner:

A crude clay containing:

34.5 per cent of total moisture 24.7 per cent of A1203 1.3 per cent of Fe203 small amounts of alkali and alkaline earth, and the remainder silica with a little Ti02, is calcined for 3 hours at 600 C., losing 34.5 per cent of its weight, in the form of water. After calcination, the composition is:

37.8 per cent of A1203 2.0 per cent of Fe2O3 1.9 per cent of Ti02 small amounts of alkali and alkaline earth, and about 58 per cent of Si02.

500 grams of this calcined and ground clay are treated with 6.4 liters of water and 1.2 kilograms of S02, in an acid-proof stirrer-autoclave, for 16 hours at 70 C. and under a pressure of about 9 atmospheres. Thereafter, the greater part of the sulfurous acid is allowed to blow off, the contents of the autoclave being cooled down, and the reaction mixture is filtered off from the undissolved residue-mainly silica--of the clay. The filtered solution contains:

2.5 per cent of A1203 0.14 per cent of Fe2O3 0.018 per cent of S102 0.00 per cent of Ti02 and traces of alkali and alkaline earth.

The yield from the decomposition is given in the following table in percentage of the substances present in the original clay:

85.0 per cent of A1203 90.0 per cent of Fe203 0.4 per cent of S102 and 0 per cent of Ti02.

Steam is blown directly into the solution, until the escape of S02 ceases. At the same time, 840

grams of a white salt separate out and are then filtered off from the solution out of contact with air. The precipitated salt has the composition:

- 19.2 per cent of A1203 7.7 per cent of S02 0.96 per cent of Si02,

the remainder being water. I

The filtrate contains all the iron and small amounts of alkali.

Example III mina in available form, a process which com-f prises treating the ground and calcined mineral in aqueous suspension at temperatures between 50 and 80 C. with S02 under pressure and in quantity such as to form a solution containing from 10 to 40 grams A1203 per liter and substantially free of silica, separating said solution from the insoluble residue and recovering alumina from the separated solution.

2. In the decomposition of siliceous aluminous minerals by sulfurous acid for extraction of I alumina in available form, a process which comprises treating the ground and calcined mineral in aqueous suspension at temperatures between 50 and 80 C. with S02 under pressure and in quantity such as to form a supersaturated aqueous solution of S02 between 10 and 50 per cent in strength.

3. In the process of recovering aluminum values from clays, kaolins, leucites, high-silica bauxites and other siliceous-aluminiferous minerals, the steps which comprise extracting such a mineral with an aqueous solution of S02, raising the temperature during said extraction by the application of external heat to values ranging from about 50 to 80 C., maintaining the S02 in sufficient excess to prevent decomposition of the aluminum sulfite solutions thereby formed, continuing the extraction until a solution is formed containing a substantial amount of A1203 but substantially free from S102, separating said solution from the insoluble residue and recovering the extracted aluminum values from the separated solution.

4. The process of claim 3 wherein the extraction is conducted at substantially atmospheric pressure, S02 being introduced into the extraction zone at least as rapidly as it is consumed.

5. The process ofv claim 3 wherein the extraction is conducted under a superatmospheric pressure of S02.

6. The process of claim 3 wherein the mineral is calcined prior to extraction for removal of water of hydration.

7. The process of claim 3 wherein the extrac tion is conducted with an aqueous solution of S02 containing not substantially less than 10 per cent and notsubstantially more than 50 per cent S02 by weight.

ERICH WIEDBRAUCK. KARL BUCI-IE. 

